1. Field of the Invention
This invention relates to thixotropic cement composition and methods of using same, and more particularly discloses a method for sealing or cementing subterranean zones penetrated by a borehole, such as cementing the annular space between an oil or gas well casing and the surrounding formation.
2. Description of the Prior Art
Oil, gas and water formed in the ground are under great pressures. Drilling into these formations requires borehole pressures to overbalance the formation pressure to prevent the uncontrolled flow of these formation fluids into the well bore. These pressures are controlled by maintaining sufficient hydrostatic pressure in the borehole. This is initially accomplished by circulating drilling fluids through these pressurized intervals.
Commonly, the first step in operations conducted to produce hydrocarbons from subterranean formations, is to cement or seal the area between the casing and the formation wall. This is accomplished via either directly introducing the cement into the space between the formation wall and the outer side of the casing or via pumping the cement into the casing with sufficient pressure to force the cement back up the annular space between the outside of the casing and the formation wall. Such initial cementing operations are referred to as primary cement.
Other types of jobs utilizing cement during the life of a well are referred to as secondary or remedial cementing. Such secondary cementing deals with completion and remedial repairs on a well after the producing zone is reached. Such activities include squeeze cementing (a procedure whereby a slurry is forced into the formation by pumping into the hole while maintaining a back pressure), plugging back with cement to shut off bottomhole water, plugging of crevices, cavities, leaks, and "thirsty" formations that cause lost circulation, and cementing casing leaks.
To described a typical well operation would not be an easy task as cementing conditions may range from very shallow to in excess of 30,000 feet. However, in all wells, two fundamental conditions are present that are not normally found in the handling and placement of concrete, i.e., temperature and pressure. Temperatures may range from below freezing in the permafrost in Alaska and Canada to 700.degree. F. in geothermal steam wells of the Salton area of Southern California. Pressures in the deep, hot wells may exceed 20,000 psi and, along the Gulf Coast, cementing pressures in excess of 10,000 psi are not uncommon. Both have their influence in the effective placement of cement beneath of earth's surface.
Cavernous or interconnected vugular zones require only that the fluid pressure in the zone be exceeded to create complete lost circulation. The most common type of lost circulation is attributed to the formation pressure parting or fracturing. Breakdown gradients of 0.60 to 0.65 psi/ft are common in many areas and weak, low-resistant formations with parting pressure less than 0.5 psi/ft are occasionally encountered. Not all formations which have low fracturing gradients result in lost circulation problems. Many formations are drilled with mud densities in excess of the fracturing pressure. Apparently the mini-fractures which are encountered promptly plug with mud solids and drill cuttings. This buildup conditions the wellbore which allows the mud pressure to exceed the normal fracturing extension pressure. Unfortunately, many zones will accept large amounts of drilling mud (or cement slurry) without plugging. Such formations act as pressure relief valves and can often result in cement loss where the critical pressure is exceeded.
Squeeze cementing is a well-known procedure in the art relating to the oil field industry. In general, squeeze cementing is utilized to attempt to obtain a positive and permanent seal between the well bore at the subterranean earth formation surrounding the well bore at a desired location. A problem frequently encountered in squeeze cementing is the loss of the slurry to the formation. In highly permeable or porous formation, a substantial portion of the cement utilized may be absorbed by the formation due to its low resistance to fluid flow thereby preventing a positive seal from being obtained.
The petroleum industry has employed thixotropic cements primarily to assist in controlling lost circulation problems, in certain squeeze cementing applications and in situations where maintaining annular fill is a problem. Cements possessing thixotropic properties are desirable since they provide rapid development of static gel strength after placement. That is to say, thixotropic cements are designed so that slurry viscosities remain low while the slurry is moving, but when allowed to remain static will rapidly gell.
An example of a thixotropic cement composition and a method of using same to seal subterranean formations is described in U.S. Pat. Nos. 3,835,926 and 3,928,052. The composition is comprised of water, hydraulic cement, a silicate compound, a hydroxide and a salt. U.S. Pat. Nos. 3,959,003 and 3,804,174 describe a cement composition that includes as an additive a complex reaction product of a water-soluble carboxyalkl, hydroxyalkyl or mixed carboxyalkylhydroxyalkyl ether of cellulose and a polyvalent metal salt. The composition exhibits thixotropic properties and the preferred reaction product uses hydroxyethylcellulose and zirconyl chloride. Unfortunately, new testing procedures have indicated that the compositions of these patents exhibit rather limited thixotropic properties.
There are several disadvantages associated with the above compositions. In many slurry designs, it is difficult to control the magnitude of static gel strength development through adjustments in additive level. Increasing the additive levels tend to prohibitively shorten the thickening time. Further, acceptable thixotropic response beyond about 200.degree. F. is difficult to achieve. Still further, acceptable thixotropic response in light weight slurries is also difficult to attain.
Accordingly, a need exists for a method of cementing oil and gas wells that provides a highly thixotropic cement slurry that develops high gel strengths in a sufficiently short time span to be useful in combating lost circulation problems.